AN2760 Application note

Transcription

1 Application note Using clock distribution circuits in smart phone system design Introduction As smart phones become more and more popular in the market, additional features such as A-GPS, Bluetooth, WLAN and DVB-H are now included in the cell phone design. In all of these application modules, there is a common need for a master clock which is typically an external crystal oscillator. With the master clock the data transmitted or received is modulated to the proper frequency. In this application note, conventional clock trees which use discrete crystals for different modules or discrete buffer solutions are compared to the clock solution using the integrated STCD1020/1030/1040 series circuits from STMicroelectronics (referenced as STCD10x0 throughout this application note, please refer to the STCD1020, STCD1030, STCD1040 datasheet). The benefits of using STCD10x0 are illustrated and technical hints are given to help cell phone system designers use the STCD10x0 clock distribution solution. July 2008 Rev 1 1/14

4 Conventional cell phone clock solutions compared to the STCD10x0 integrated solution AN Conventional cell phone clock solutions compared to the STCD10x0 integrated solution 1.1 Conventional clock solution - dedicated crystals for dedicated modules The easiest design of the clock tree in smart phones is to use dedicated crystals for different application modules. This is illustrated in Figure 1. Figure 1. Conventional clock solution - dedicated crystals TCXO XO XTAL WLAN Bluetooth Media Media ai14013 Although the solution in Figure 1 is simple, an obvious drawback is the cost. Normally WLAN and Bluetooth can accept the master clock in several different frequencies. For example, 13, 16, 16.8, 19.2, 26, 3.6, 38.4 and 52 MHz are all accepted by WLAN, Bluetooth, and FM radio. Different modules recommend different master clock frequencies and those recommended values are not always the same. For example, 19.2 MHz is recommended for Bluetooth by some chip vendors while 26 MHz is recommended for WLAN. Due to the design risk and time-to-market, cell phone vendors are not willing to use the same crystal to serve all application modules although this is possible. A typical external clock source (typically 10 pf load capability) is always short of fan out capability if several application modules are directly connected to its output. 1.2 Conventional clock solution - discrete buffer solution Since the cost of a TCXO (temperature compensated crystal oscillator) is extremely high and some of the common frequencies can be used for different applications, cell phone designers have tried to use a discrete buffer solution (with emitter followers) to solve the insufficient fan out issue. A two-stage discrete buffer is used as shown in Figure 2. The first stage could solve the problem of the fan out of the clock source (normally TCXO) and also provide a good isolation of input channel to output channels. The second stage distributes the master clock to different application modules and also provides good isolation between different output channels. 4/14

5 Conventional cell phone clock solutions compared to the STCD10x0 integrated solution Figure 2. Conventional clock solution - discrete buffer solution 2.8V C602 R603 R627 C603 CLK_IN R602 R601 V601 C602 CLK_OUT Clock Source ai14014 In Figure 2 only one clock source (TCXO) is used in order to greatly reduce the cost, but there are several drawbacks. The most important issue is the PCB area. This solution includes 5 discrete buffers and each buffer consists of many discrete components such as a bipolar transistor, resistors and capacitors. The large PCB area and the difficulty in routing always become problematic for the system designers. The second issue is the large quiescent current (the static current consumed by the buffer itself) and the inability of the output buffers to be shut down (constantly conduct quiescent current) to save current even when the connected application module does not require the clock. 1.3 Integrated clock solution - STCD10x0 In order to resolve the drawbacks of conventional clock solutions and facilitate cell phone system design, STMicroelectronics has introduced an integrated clock solution with STCD10x0 series clock distribution circuits. A typical application is given in Figure 3. The user should note that both sine wave and square wave clock source can be used and the STCD10x0 can distribute to the outputs with unity gain clocks. 5/14

6 Conventional cell phone clock solutions compared to the STCD10x0 integrated solution AN2760 Figure 3. Typical application circuit using STCD1040 in mobile phone or MID application V CC BT_External_Req VCTCXO MCLK V CC STCD1040 EN4 CLK4 EN3 CLK3 EN2 CLK2 Internal_Req Bluetooth WLAN GPS GND EN1 CLK1 Other device ai14015 Note: The output DC cut capacitors should only be used on the sine wave clock transmission (not necessary for square wave clock source). An external clock source (VCTCXO: voltage controlled TCXO) can be connected to the STCD10x0 directly since a DC-CUT capacitor has been integrated into the STCD10x0. Each of the output channels can be enabled/disabled individually so that there is no extra current consumption when there is no clock request for this channel. In order to match different application requirements, two voltage versions (1.8 V and 2.8 V) can be selected by the users. Four-channel outputs with 10 pf capacitive loads for each output consume only 2.8 ma quiescent current. A comparison of the STCD1040 and a discrete buffer solution is given in Table 1. Table 1. Conventional discrete buffer and integrated STCD10x0 clock solution Parameter Conventional discrete buffer STCD10x0 PCB area/cost Large Small (up to 60% less PCB area) Quiescent current High Low (at least 30% less current) Output enable Always on Can be disabled individually Input DC-CUT cap Required Not required System cost High Low 6/14

7 Conventional cell phone clock solutions compared to the STCD10x0 integrated solution 1.4 Integrated STCD10x0 clock solution in MID application Mobile Internet Device (MID) represents a new category of small and truly mobile devices which integrate the advantages of both UMPC (Ultra-Mobile PC) and mobile phones. This application has recently gained much attention and the trend to incorporate many types of wireless connection technologies such as WLAN, BT and GPS into MIDs. The STCD10x0 solution provides significant benefit to the MID clock solution, not only in system cost but also in PCB area. The connection of the STCD10x0 clock solution in MIDs is also illustrated in Figure Integrated STCD10x0 clock solution in multimode RF frontend In multimode mobile RF front-end, multiple reference clocks are typically used for the transceivers. Figure 4 gives an example of TD-SCDMA/GSM dual-mode mobile RF frontend clock application. Alternatively, the clock from VCTCXO could be distributed by STCD10x0 to the TD-SCDMA transmitter and receiver and GSM transceiver separately. Figure 4. Typical application circuit using STCD1040 in dual-mode mobile RF frontend V CC Mode selection VCTCXO MCLK V CC STCD1040 EN4 CLK4 EN3 CLK3 EN2 CLK2 TD-SCDMA transmitter TD-SCDMA receiver GND EN1 CLK1 GSM transceiver ai /14

8 Cell phone system design with STCD10x0 clock solution AN Cell phone system design with STCD10x0 clock solution 2.1 Preparation to use STCD10x Setting a common clock frequency for each application In each of the applications (WLAN, GPS, BT, FM radios etc.), the default reference clock frequencies are not the same. The user should configure the internal register or external pins of each application module to make sure they accept the same clock frequency. Once this is done, the common clock source signal can be distributed by the STCD10x0. Table 2 gives the user an example of different methods to configure each application module to accept the same common reference clock frequency. CS is the GPS chip from CSR, GSC3KT is the Bluetooth chip from Sirf, STLC4550 and STLC9000 are WLAN chips from STMicroelectronics. Normally, 19.2 MHz, 26 MHz and 38.4 MHz are commonly accepted and the user can configure the internal registers, or external pins, or just directly connect to ensure they run at the same common frequency. Table 2. Part Configuring common reference clocks for GPS, BT or WLAM Common ref. freq. Ref. clock setting method How to configure CS (CSR) GSC3KT (Sirf) STLC4550 (ST) STLC9000 (ST) 19.2 MHz Set registers "PSKEY_ANA_FREQ (0x1fe)" and "PSKEY PLLX_FREQ_REF (0xabc)" value to "19200". 26 MHz Internal register Set register "PSKEY_ANA_FREQ (0x1fe)" value to "26000" MHz 19.2 MHz Set registers "PSKEY_ANA_FREQ (0x1fe)" and "PSKEY PLLX_FREQ_REF (0xabc)" value to "38400". Set pins "ED[7]:JTCK:JTDI" to be "010". 26 MHz Pin configure Set pins "ED[7]:JTCK:JTDI" to be "110" MHz Set pins "ED[7]:JTCK:JTDI" to be "000" MHz Set pins "MODE(4:0)" to be "00100". 26 MHz Pin configure Set pins "MODE(4:0)" to be "11100" MHz Set pins "MODE(4:0)" to be "11010" MHz 26 MHz 38.4 MHz Automatic detection An integrated automatic detection algorithm detects the system clock frequencies 8/14

9 Cell phone system design with STCD10x0 clock solution Choosing the right clock source Phase noise is a critical specification in reference clocks. The total phase noise of the clock tree should be obtained by summing the additive phase noise of the STCD10x0 and the phase noise of the clock source (TCXO) in terms of power as illustrated in Equation 1: Equation 1 PN T = 10 PN 10 C + 10 PN 10 X < PN A where: PN T is the total phase noise in dbc/hz PN C is the additive phase noise of STCD10x0 and PN X is the phase noise of clock source Make sure the total phase noise is kept within the phase noise requirement of each application PN A. The user should choose the right TCXO with proper phase noise to meet the requirement. 2.2 Power supply V CC of STCD10x0 and decoupling The STCD10x0 is designed with ultra-low added phase noise so that the device can also be used for master clocks in RF front-end transmitters and receivers for cell phones. A very clean power supply ( as specified 30 µvrms supply noise from 300 Hz to 50 khz in the datasheet) is needed. Normally a TCXO (or VCTCXO) is used as the master clock source. It is strongly recommended to share the same power supply for STCD10x0 with the TCXO since the supply for the TCXO is always an ultra-low noise, high PSRR, LDO. The LDO can be shut down when the TCXO (also STCD10x0) is not needed. A 1 µf capacitor is highly recommended to be placed as close as possible to the V CC pin of the STCD10x0 on the PC board to minimize the noise of the V CC and guarantee the performance of the phase noise. 2.3 Connection of the MCLK pin to external clock source If the output of the clock source voltage level is within the supply rails of the STCD10x0, the output of the source clock should be connected directly to the MCLK of the STCD10x0. The direct connection of the source clock is the most common case, allowing a DC-CUT capacitor to be saved on the PCB. The input clock voltage level of the STCD10x0 cannot exceed the supply rails when it is directly connected to the source clock. If it is necessary to connect a source clock with a voltage level exceeding the supply rails of the clock distribution circuits in the application, the user needs to connect a DC-CUT capacitor serially as shown in Figure 5. A voltage divider formed by a resistor string is also needed to set a proper DC bias for the clock input which can keep the clock voltage within the supply rails of STCD10x0. The proper DC voltage is around half of the supply. 9/14

10 Cell phone system design with STCD10x0 clock solution AN2760 Figure 5. Connection of the DC-CUT capacitor and bias V CC R 1 VCTCXO OUT 0.1μF DC-CUT R 2 MCLK STCD1020 STCD1030 STCD1040 ai Connecting STCD10x0 outputs to application modules The output channels of the STCD10x0 are biased at half of V CC internally by design. 1µF DC-CUT capacitors are required at each of the outputs in most applications. 2.5 Power-down sequence If the STCD10x0 shares the same power supply with TCXO (clock source), it is suggested to power up the TCXO before enabling each of the clock outputs since normally it will take several milliseconds for the TCXO to stabilize its output. If the STCD10x0 uses different power supplies with TCXO, it is not recommended to power down V CC of STCD10x0 since it only consumes standby current which is no more than 1µA (when no clock output is enabled, the STCD10x0 enters standby mode). If shutdown of the STCD10x0 is preferred, the following power-down steps must be followed: 1. Shut down clock source TCXO output 2. Disable EN pins, pull low 3. Shut down STCD10x0 power V CC If the power-down sequence is not strictly followed as above, the user may find the clock output waveform viewed at the condition of TCXO active with output, V CC of STCD10x0 shutdown, and EN pins active. This possibility is illustrated in Figure 6. 10/14

11 Cell phone system design with STCD10x0 clock solution Figure 6. Power-down possibility (incorrect sequence) In the STCD10x0 chip design, there are ESD diodes for the EN pins to connect to V CC and GND for ESD protection. If V CC is powered down (disconnect to supply) and an active high signal is present on the EN pin (for example, 2.8 V), the ESD diode which connects the EN pin to V CC will conduct and pull V CC up to 2.1V. When the external TCXO is active and sends a clock output to MCLK of STCD10x0, a clock waveform should be viewed at the output as illustrated in Figure 6 which could cause a problem to the application. 11/14

12 Conclusion AN Conclusion As smart phones become more and more powerful with versatile application features, more and more clocks are needed in the cell phone system design. Conventional clock solutions with dedicated crystals or discrete buffers cannot meet the requirement of the design. Using the STCD10x0 in cell phone system design is a very cost-effective, easy solution and shortens the design and layout cycle which allows for faster time-to-market. 12/14

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